Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

331
FXUS63 KLOT 130621
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
121 AM CDT Mon Jul 13 2026

.KEY MESSAGES...

- Hot conditions are expected for much of the upcoming week;
  though somewhat cooler near Lake Michigan.

&&

.DISCUSSION...
Issued at 121 AM CDT Mon Jul 13 2026

The core of a nearly 600 dam 500 mb high will start to shift
east across the northern Plains/upper Mississippi Valley today
and will eventually become nearly stationary across the region
through the week. Column thicknesses will commensurately
increase today leading to high temperatures taking a roughly 3
to 9 degree jump over those observed yesterday. With a general
lack of deeper moisture, cut-off Gulf trajectories, and strong
mixing today, have blended some of the lower dewpoints from the
drier guidance (HRRR/RAP). This results in peak heat indices
generally in the mid 90s away from the lake today.

Locally, 1000-500 mb thickness values are forecast to peak on
Tuesday and Wednesday, resulting in the hottest conditions of
the week. Could see highs making a run into the upper 90s for
locales just west of the inland-advancing lake breeze during
peak heating, which includes O`Hare, Midway, and parts of the
western Chicago suburbs. Dewpoints remain a lingering
significant area of uncertainty during this upcoming stretch,
which continues to lead to somewhat lower-than-typical
confidence in peak heat indices. Unlike the previous heat wave,
moist Gulf trajectories will remain completely blocked off in
our region as a persistent surface high will be present just to
our west. On the other hand, we`re nearing the time of peak
evapotranspiration (ET), with crops releasing vast amounts of
water vapor into the atmosphere. These two effects remain at
odds with one another, as a lack of deep lower-tropospheric
moisture combined with strong mixing should allow dewpoints to
otherwise mix out during the afternoon.

Based on the latest multi-model consensus, and taking into
account the potential for at least some influence of increased
ET, will show dewpoints Tuesday afternoon pushing towards 70 away
from Chicago, and then a bit more solidly into the low 70s on
Wednesday. This results in peak heat indices around 100 on
Tuesday and then possibly nearing 105 degrees on a localized
basis on Wednesday. Trends today may serve as a bit of a litmus
test regarding forthcoming values, as we`ll get a chance to see
if dewpoints match the typically moist-biased local boundary
layer parameterization schemes such as the NAM/NSSL WRF, etc or
the drier and stronger mixers like the RAP and HRRR.

Daily lake breezes will afford modest relief, but only right at
the lake as the cooling effects will be muted by warm lake
temps (mid 70s) and limited inland movement.

On Thursday, the GFS, NAM, and UKMET all weaken the ridge
sufficiently to allow a backdoor cold front through the region--
although a notable slowing trend occurred in the latest 00z GFS
run. The CMC, while not nearly as substantial with the synoptic
boundary, allows of much more robust lake breeze passage during
the afternoon. The ECMWF remains a general outlier, holding
onto offshore flow through the end of the week with a notably
hotter solution even into Saturday. This model spread results in
increasing forecast uncertainty at the end of the week.

At this time, there are no plans for heat headlines. If
dewpoints end up meaningfully higher than advertised, that could
change, although in that scenario, air temperatures would likely
be a few degrees lower. Cook County could remain the exception
in the event we reach the three-day 100 F heat index criteria,
but given the potential for more substantial lake cooling on
Thursday, there are also no plans for an Extreme Heat Watch for
the city at this time.

Late this week and into next weekend, medium range guidance is
in pretty good agreement in breaking down the upper ridge with
more of a northwest flow pattern setting up locally. This could
result in a more active pattern with chances for showers/storms
increasing late in the week into next weekend.

Carlaw

&&

.AVIATION /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1210 AM CDT Mon Jul 13 2026

VFR and light winds are expected through the forecast period.
Winds overnight will be light and variable if not outright calm.
After sunrise, they are expected to remain below 10 knots but
prevail out of the northwest. A lake breeze is expected to
develop in the afternoon, which may switch wind directions to
the east-northeast for terminals closer to the lake, but there
is uncertainty on the inland extent for the wind direction
change.

For situational awareness: there is a storm complex presently
crossing the Upper Peninsula of Michigan diving southward toward
the rest of Michigan. No precipitation is expected, but there
is a non-zero chance this complex sends outflow westward that
could flip wind directions to the east for a short period later
this morning. Confidence is too low for any mention in the TAF.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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